The present invention relates generally to the maintenance of high pressure steam generating heat exchangers suitable for use in conjunction with the generation of electricity at nuclear powered facilities. More particularly, the present invention is related to a non-chemical, non-corrosive cleaning process for tube bundle heat exchangers, which must be cleaned at periodic intervals for the removal of accumulated sludge and debris.
The prior art reflects a variety of steam generation systems for use at nuclear powered electrical generation plants. All modern nuclear steam generators generally exchange heat between a reactor-heated primary fluid and a secondary fluid such as water which flows through the steam generator in isolated, heat exchange relation with respect to the primary fluid. The various types of heat exchangers used in nuclear power generation systems, as will be recognized by those skilled in the art, may broadly be characterized as being of the recirculating type or the "once through" (OTSG) type. In either case a primary and secondary coolant are disposed in heat exchange relationship, and necessary operating temperatures and desired energy levels of operative steam are facilitated via appropriate circulation through the apparatus. The present invention is adapted to be successfully employed in conjunction with cleaning of either once through steam generator heat exchangers (OTSG's) or recirculating heat exchangers.
U.S. Pat. No. 4,158,387 issued to the Babcock and Wilcox company on June 19, 1979, discloses a once through steam generator (OTSG) for use at nuclear power plants. The device comprises an upright pressure vessel having disposed therewithin a plurality of heat exchange tubes which extend longitudinally through the interior of the apparatus and which are braced and supported at vertically spaced apart intervals by a plurality of suitable tube bundle support plates. The heat exchange tubes are confined within a generally cylindrical shroud concentrically disposed within the generator, and the space between the shroud and the outer casing is divided into an upper steam annulus and a lower downcomer annulus. A first heat exchange passageway to the system is defined through a primary coolant inlet nozzle which thermally communicates with a plurality of elongated, longitudinally extending heat exchange tubes, and terminates in a suitable outlet channel for transmission of the primary coolant back to the reactor.
Concurrently, incoming water to be vaporized is transmitted inwardly of the downcomer annulus to near the vessel bottom, through various windows, and upwardly through the tube bundle shroud between the spaces defined between adjacent tubes and the tube bundle support plates. Significant boiling occurs, and steam generated thereby exits the generator through the steam annulus. A similar heat exchanger is seen in U.S. Pat. No. 4,068,627 issued Jan. 17, 1978.
However, as will be well recognized by those skilled in the art, the use of such generators results in the accumulation of certain undesirable solid contaminants which are deposited within the heat exchange apparatus, primarily upon the tube sheets, tubes and their tube support structures. Entrained and dissolved solids, most of which are introduced into steam generators by entering feed water, tend to concentrate within the boiler water rather than exiting with the generated steam. A significant quantity of entrained and dissolved solids will be distributed throughout the boiler water. Their accumulation upon the tubes and the various tube bundle support plates will interfere with secondary coolant transmission and cause the reduction of heat transfer efficiency. Corrective maintenance is thus periodically mandated.
Within the once through steam generator (OTSG) particularly vexatious sludge accumulates upon the entrance and flowholes of the tube bundle support plates. This deleteriously increases flow resistance for the secondary fluid, thereby increasing the water level in the downcomer due to the flow resistance, eventually requiring a reduction in plant operating capacity to prevent flooding of the steam aspiration port and concomitant generator instability. It has also been suggested that particles of sludge broken loose from internal surfaces (for example tube scale) by normal operating transients may be carried upwardly by the steam flow such that they can block or become wedged in the flow holes of upper tube support plates and cause an increase in flow resistance. In a recirculating heat exchanger contaminant buildup can cause tube plate cracking, tube denting or other undesirable conditions.
Contaminant removal is thus mandatory for efficient steam generator operation, and a variety of solutions have been proposed previously for removing unwanted contaminants. U.S. Pat. No. 4,158,387 discloses relevant background information which is extremely relevant to the present invention. It provides a blowdown apparatus suitable for use with OTSG heat exchangers whereby sludge accumulation and similarly unwanted debris may be disposed of as a means of cleaning. A related blowdown system for sludge cleaning is suggested through the use of reverse circulation and the injection of nitrogen gas by the apparatus of U.S. Pat. No. 4,261,300.
U.S. Pat. No. 2,972,986 discloses a system wherein the concentration of dissolved solids within a steam generator contemplates the discharge or blowdown of unwanted water. A steam generator with vertical tube sheets is disclosed in U.S. Pat. No. 4,068,627, wherein it is provided for the collection of the unwanted crud or solid deposits immediately above the lowest tube sheet. A liquid metal steam generator is disclosed in U.S. Pat. No. 3,888,212. Similarly, a steam generator having a U-shaped tube bundle is seen in U.S. Pat. No. 3,942,481.
A nuclear steam generator which has a blowdown pump arranged to pump water from a blowdown line is disclosed in U.S. Pat. No. 4,261,300. U.S. Pat. No. 3,895,465 issued July 22, 1975 contemplates the injection of particles of Boron Trioxide propelled through suitable jets with compressed gas to cause appropriate abrasion whereby contaminated surfaces within the nuclear heat exchanger may be appropriately cleaned.
It is well known to provide internal cleaning through various chemical additive systems, wherein the injection of certain corrosive chemicals within the heat exchanger chemically dislodge unwanted solids. Typical of this process is the system described in U.S. Pat. No. 3,900,010 issued Aug. 19, 1975.
However, chemical cleaning systems of the type known to us inherently give rise to a variety of problems. Conventional chemical cleaning systems are rather slow and tedious to employ. Also, the injection of suitable chemical additives of appropriate strength to dissolve, dilute or dislodge accumulated sludge and debris often leads to unwanted deleterious damage within the apparatus being cleaned. Moreover, disposal of the used chemicals is difficult and costly.
Accordingly, the prior art has suggested the use of various agitation systems including gas injection for cleaning the interior of heat exchangers. Gas injection has been employed previously in conjunction with U.S. Pat. No. 4,261,300 issued Apr. 14, 1981 for the purpose of agitating the water during wet lay up to improve distribution of recirculated water in an attempt to more conveniently and economically extract unwanted solids. This may be used in conjunction with blowdown, recirculation and filtration.
Recently it has been proposed to clean tube bundle heat generators (OTSG's) by periodically subjecting the tube bundle shroud interior to blasts of high pressure gas. Such systems periodically inject nitrogen through suitable radially spaced apart pulsers to periodically release high pressure bubbles of nitrogen. The sudden release of the high pressure gas produces a pressure pulse such that explosive noncontinuous spherical shock waves agitate the deposited sludge to provide a degree of cleaning. It is believed that ANCO Engineers Inc. of Culver City, Calif. have previously proposed gas injection involving sonic cleaning of submerged parts within the interior of OTSG's in U.S. patent applications Ser. Nos. 06/742,134 (a continuation of prior patent application Ser. No. 06/486,352); 06/686,242 and 06/604,048.
In our opinion, however, and based upon our own test experience and data, mere sonic shockwave agitation insufficiently cleans the critical clogged holes in the tube bundle support plates of conventional OTSG's, and the shock waves must be limited in intensity to prevent deleterious effects upon critical steam generator components.
For vigorously cleaning the interior of tube bundle heat exchangers, we have found it desirable to utilize the energy of controlled high pressure gas injection at discrete intervals to periodically lift the pool of water such that the rising surface of the water periodically slaps the surface to be cleaned. The phenomena is referred to herein by the term "water slap." The process intentionally reduces the acoustic energy content of the water column by slowing the gas release rate to eliminate potentially damaging effects.
Specifically, the water surface impact upon the tube support plate produces very high localized pressures which are sufficient to break up and dislodge deposits in the path of the advancing water surface. These pressures and the resultant high velocity flows of the ascending water level created by the introduction of inert gas into the bottom of the heat exchanger are thereby concentrated on the desired vessel internal.